Let musb_hdrc on DaVinci use the clock framework, now that it
seems to work right.

 * add USBCLK to dm644x support (it's already there for dm355)
 * only enable it when the driver asks for that
 * make the musb_hdrc driver use that clock

The original failures seem to have been caused by really buggy code
implementing the clock APIs.  Now they're working right.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>